Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1999-11-17
2001-08-28
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S602000, C073S626000, C073S628000, C600S443000
Reexamination Certificate
active
06279397
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of using ultrasonic waves to non-destructively detect structural characteristics of work pieces. More particularly, this invention relates to a method and apparatus for focusing ultrasonic waves that are propagated from a phased array to a spherically-bounded object so that the waves arrive at a pre-selected focal point at the same time and in phase and thereby significantly improve, nondestructively, the detectability of possible flaws in and structural characteristics of the object. Still more particularly, this invention relates to a method and apparatus for non-destructively evaluating spherically bounded objects, such as reactor pressure vessels, ball bearings, and the like by obtaining by iteration of a focal law equation according to the invention which follows a path on the surface of the spherically bounded object.
BRIEF DESCRIPTION OF THE PRIOR ART
The use of ultrasonic waves generally, and phased array mechanisms in particular, to nondestructively detect structural characteristics of work pieces is well known in the art. Generally, the technique consists of transmitting ultrasonic waves toward the work piece through various media such as water or air so that the waves impinge the surface of the workpiece, propagate throughout the internal structure of the work piece, and ultimately reflect back from the work piece. As the waves propagate throughout the work piece, they are reflected and refracted by variations and changes in the medium through which they travel. Defects in the structure of the work piece affect the travel path of the propagating waves. When the propagating waves reflect from the work piece, they are measured and analyzed. The structural characteristics of the work piece, including any defects or flaws, can be detected and reconstructed from the information contained in the reflected waves.
A phased array mechanism transmits ultrasonic waves from its multiple array elements which are spaced apart from each other. The waves are transmitted in a sequence at slightly different times relative to each other. The transmitted waves thus travel through a coupling material, which is usually water, prior to impinging the surface of the target work piece. Because of the molecular differences between the coupling material and the work piece, a portion of the propagating wave is reflected away from the work piece and a portion is refracted into the work piece. It is the refracted portion of each propagating wave that is used to detect and reconstruct structural defects and flaws of the work piece. The point at which the propagating waves impinge the work piece relative to their respective origination points partially determines the path of the refracted waves as they travel within the work piece.
In order to have the greatest ability to detect and reconstruct structural characteristics of the work piece, it is desirable for the refracted waves that enter and later exit the work piece to have the greatest possible amplitude. Waves that exit the work piece with a greater relative amplitude provide stronger and more readable signals.
It is well known that waves which arrive at a particular focal point at the same time and in phase constructively interfere with each other to create a wave with a larger relative amplitude. Accordingly, to improve the detection and reconstruction capabilities of phased arrays, it is desirable to focus all of the waves emitted from the elements of the phased array mechanism to create an internal wave with the greatest possible amplitude. To do so requires that all of the waves be focused and sequenced so that they arrive at a pre-selected focal point at the same time and in phase. However, until this invention, there was no way to determine the preferred refraction point on a spherical work piece or the proper sequence of transmitting pulses from the phased array so that the propagating waves arrived at a selected focal point at the same time and in phase in a work piece having a spherical boundary.
Accordingly, it is a continuing aim in the art to provide, in the field on nondestructive testing, a method and apparatus for determining the proper sequence of transmitted pulses in a phased array and a method and apparatus for visualizing the propagating wave paths in a spherically bounded material and contoured material that is spherical in the region being evaluated.
SUMMARY OF THE INVENTION
This invention comprises a method and apparatus to focus ultrasonic pulses transmitted from a phased array mechanism into a spherically-shaped work piece to increase the effectiveness of the non-destructive internal examination of the work piece. While this invention can be used with any spherical object, one such possible use is in connection with detecting internal flaws in nuclear reactor pressure vessel heads. To maximize the strength of the ultrasonic pulses when the pulses arrive at a particular selected focal point in the work piece, each pulse transmitted from an element of the phased array should be properly sequenced so that all of the pulses arrive at some selected focal point at the same time and in phase. In one aspect, the invention thus relates to a method and apparatus to determine to where the penetrating waves propagate to in the spherical work piece and how to properly sequence the transmitted pulses from all the phase elements comprising the phased array.
The method of the invention includes steps of determining location coordinates for preferred refraction points on the surface of the work piece that will direct impinging waves to arrive at a pre-selected focal point simultaneously. First, a focal point is pre-selected, and, accordingly, the location coordinates for the focal point are known before the method according to the invention is used. Second, the location coordinates for each element of the phased array are also known before the method according to the invention is used. Based upon the coordinates of the elements of the phased array mechanism and the pre-selected focal point, the coordinates for a desired refraction point on the surface of the work piece are calculated for each element of the phased array. In addition, a unique pulse firing time for each element of the phased array is calculated. The unique refraction point and pulse firing time for each array element permit the phased array to transmit the pulses from each element so that they all arrive at the focal point in phase and at the same time.
Each refraction point is uniquely determined relative to the center of the work piece by calculating two angles, &thgr;
p
and &phgr;
p
, which together define a unique point on the surface of the work piece. The method of this invention includes an iterative calculation to determine &thgr;
p
and &phgr;
p
. The values of &thgr;
p
and &phgr;
p
are referred to as &thgr;
p
and &phgr;
p
for each i
th
iteration of the method The method according to the invention begins by setting &thgr;
i
to an initial value between 0 and 360, preferably to 0. Based upon the initial selected value of &thgr;
i
, the angle created by the line extending between the center of the work piece and the particular element of the phased array on the one hand, and the line extending between the center of the work piece and a potential refraction point on the other hand, is calculated. This angle is referred to as &bgr;. Then, the angle created by the line extending between the center of the work piece and the potential refraction point on the one hand, and the line extending between the center of the work piece and the pre-selected focal point on the other, is calculated. This angle is referred to as &bgr;
f
. New values for &thgr;
p
and &phgr;
p
are calculated based upon the values of &bgr; and &bgr;
f
. The calculation of the new values for &thgr;
p
and &phgr;
p
require the analysis of several potential special case situations. The entire method is reiterated using a newly calculated initial value for &thgr;
i
. The iteration process ceases when the value of
Kananen Ronald P.
Larkin Daniel S.
Rader Fishman & Grauer
Saint-Surin Jacques M.
Westinghouse Electric Company LLC
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